1. Technical Field
The present application relates generally to a communication system and, more particularly, to an apparatus and method for generating a pilot signal for performing a hard hand-off in a code division multiple access (CDMA) cellular system using an External Pilot Transmitter (EPT).
2. Description of the Related Art
In a cellular mobile telephone system, the cellular service area is divided into a plurality of sub-areas (i.e., cells), and each cell has a base station associated therewith. In a cellular communication system, a single mobile switching center (MSC) controls all base stations, and allows a mobile station to continue communication when the mobile station travels between several service cells. In a code division multiple access (CDMA) cellular communication system or a personal communication service (PCS) system, various types of hand-off operations are utilized to ensure continuous communication when the mobile station travels between cells (i.e., when a mobile station travels from a xe2x80x9csource base stationxe2x80x9d to a xe2x80x9ctarget base stationxe2x80x9d).
One type of hand-off operation is referred to as a xe2x80x9chard hand-off.xe2x80x9d During a hard hand-off operation, when a mobile station enters a target cell (or target base station), communication with the source base station is terminated, and then communication with the target base station is established. The hard hand-off process is completed within a very short time and the user of the mobile station does not even recognize the temporary termination of communication.
In a CDMA cellular system, frequency assignment to the various cells is typically offset due to the unbalanced distribution of subscribers. For instance, downtown city areas generally require more traffic capacity to service a large number of subscribers, whereas suburban areas require relatively less traffic capacity due to a smaller number of subscribers. Consequently, when a target base station (in which a mobile station enters) does not have a frequency assignment through which communication is currently established, or when, if any, the frequency assignment does not have enough traffic channels, a hard hand-off cannot occur.
A hard hand-off is generally performed as follows. The source base station continuously measures the signal strength of a mobile station within its cell region to determine if the signal strength drops below a predetermined threshold value. When the received signal strength falls below the threshold, the source base station determines that the mobile station is located at the boundary of its cell region, and then signals a base station controller (BSC). The BSC then decides which base station (i.e. target base station) receives a relatively strong signal from the mobile station.
When it is determined that a particular target base station (e.g., a neighboring base station) receives a strong signal, the BSC transmits a hand-off request message to the target base station, as well as a command to the mobile station to communicate with the target base station (neighboring base station). The mobile station then performs a hand-off and, accordingly, communication between the mobile station and the target base station is established.
The BSC determines whether to perform a hand-off based on a pilot signal strength of the source base station which is in current communication with the mobile station. In the conventional system, a call will be disconnected if the hand-off to the target base station is unsuccessful. There are various reasons for a hand-off operation to fail. For instance, a hand-off operation can fail if there are no available channels in the target cell for communicating the call or if the mobile station fails to receive a hand-off message. A hand-off can also be requested when a mobile station enters a shadow area of the cell area in which the pilot signal strength becomes weak. Furthermore, it is very difficult to determine a hand-off determination parameter and time in the varying cellular communication environment. And there is trade-off between coverage areas to reduce the possibility of success to perform a hand-off.
A conventional method for performing a hard hand-off using a pilot signal is disclosed in U.S. Pat. No. 5,594,718 entitled xe2x80x9cA Method and Apparatus For Providing Mobile Unit Assisted Hard Handoff From A CDMA Communication System To An Alternative Access Communication System.xe2x80x9d This method uses a pilot beacon for generating an identifying pilot signal corresponding to a target base station in order to overcome the above difficulties.
Referring now to FIG. 1, a diagram illustrates a conventional pilot signal generator for generating a pilot signal for performing a hard hand-off in accordance with the prior art. Each base station has a RF path unit, which includes a plurality of digital MODEMs 200, 210 and 220 for converting audio frequencies into intermediate frequencies, a plurality of transceivers 300, 310 and 320 for converting the intermediate frequencies into radio frequencies and a plurality of power amplifiers 400, 410 and 420 for amplifying the RF signals which are transmitted from an antenna. The digital MODEMs 200 and 210, transceiver 300 and 310, and power amplifiers 400 and 410 are utilized for communication, whereas a pilot signal generator (which generates an identifying pilot signal corresponding to a target base station) includes the digital MODEM 220 for generating a pilot signal, the transceiver 320 and the power amplifier 420. As a mobile station moves to a target base station, the mobile station simultaneously receives a weak pilot signal from the source base station and a relatively strong pilot signal from the target base station. Accordingly, the mobile station will request a hand-off and the digital MODEM 220 of the pilot signal generator will only transmit the overhead channel such as, for example, a pilot, synchronization, and paging.
The transmission of the overhead channel (as described above) causes an unbalance of coverage areas of multiple frequencies, and this unbalance increases the load of the primary frequency. Specifically, when the coverage area of the pilot signal generator is greater than that of the forward link of the primary frequency, a mobile station will more frequently request a hand-off in a base station having a pilot signal generator than in the balanced base station where the coverage area of the primary frequency is almost balanced with that of the pilot signal generator. Accordingly, a call will more frequently be handed down to the primary frequency, which results in an increase of the load of the primary frequency and reduces the performance of the entire system. A pilot digital gain modulation can control the unbalance of the coverage areas, but it is very difficult to modulate a pilot digital gain taking into consideration the actual radio environment and system operation.
In addition, when the pilot signal generator is added to the base station, it requires space for installation; a digital hardware MODEM, and a channel card. The digital hardware MODEM is used for a hand-off (not for actual communication), and for generating some channels such as a pilot channel, synchronization channel and paging channel. This pilot signal generator is not portable because it is installed in an outdoor device.
There are various disadvantages associated with the conventional pilot signal generator. For example, the pilot signal generator is physically large and significantly costly due to the large space required for installation. Moreover, the conventional pilot signal generator is not portable and requires a significant amount of labor to install it, or extract it, from the system. In addition, as stated above, it causes unbalance in coverage areas of multiple frequencies.
It is an object of the present invention to provide a method and apparatus for generating a pilot signal for a hard hand-off using an IF amplifier/divider without having to add the pilot signal device to frequency allocation generating hardware, the IF amplifier/divider being easily coupled to a digital MODEM.
It is another object of the present invention to provide a method and apparatus for generating a pilot signal to produce output to every channel in order to balance the system load.
It is a further object of the present invention to provide an apparatus for generating a pilot signal which does not require physical space and is portable.
In one aspect of the present invention, a pilot signal generator of a base station for generating an identifying pilot signal corresponding to a target base station in order to perform an inter-frequency hard hand-off operation in a code division multiple access cellular communication system, comprises an intermediate frequency (IF) amplifier/divider for dividing an intermediate frequency (IF) signal which is received from a digital MODEM; a service radio frequency (RF) path unit for up-converting a first portion of the divided signal into a radio frequency and transmitting the radio frequency, the service RF path unit being utilized solely for actual communication; and a radio frequency (RF) path unit for up-converting a second portion of the divided signal into a radio frequency and transmitting the radio frequency where the RF is used as an identifying pilot signal corresponding to the target base station for a hard hand-off, the RF path unit being used solely for transmission of a pilot signal.
In another aspect of the present invention, a method for generating a pilot signal for performing an inter-frequency hard hand-off in a CDMA cellular communication system, where a base station uses an identifying pilot signal corresponding to a target base station for performing an inter-frequency hard hand-off operation, includes the steps of: generating an intermediate frequency signal for communication by a digital MODEM; dividing the intermediate frequency signal to multiple paths; transmitting the up-converted signal to a service RF path unit for communication after up-converting a first portion of the divided signal; and transmitting the up-converted signal to at least one RF path unit in order to generate a pilot signal of a target base station after up-converting a second portion of the divided signal.
In yet another aspect of the present invention, a method for generating a pilot signal of a target base station for performing an inter-frequency hard hand-off operation in a CDMA cellular communication system includes the steps of: transmitting, at a RF path unit, all signals converted by an overhead channel or traffic channel, where the RF path unit transfers a target base station pilot signal for a hard hand-off; and transmitting, at a RF path unit, the up-converted signal after up-converting a signal of a path for a primary frequency.
These and other objects, features, and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments, which is to be read in connection with the accompanying drawings.